Kinetics and mechanisms of metal hydrides formation - A review

The microscopic mechanisms which may control the rate of the reaction of gaseous hydrogen and hydride-forming metals are reviewed. A distinction is made between the early stages of the reaction associated with the nucleation and growth of the hydrides on the surface of the reacting metal and the subsequent massive stage. For the very early stage, factors affecting the ability of hydrogen gas to penetrate surface passivation layers are considered. Different types of nucleation groups are demonstrated. For the latter, massive stage, possible morphological forms of the hydride phase development are summarized. A special case, frequently encountered in binary metal-hydrogen systems, is the contracting envelope (or shrinking core) morphology. For this case, a simple evaluation of the reaction front velocity can be deduced from the overall rate measurements and from the known geometry and dimensions of the metal sample. A detailed analysis of this hydride-front velocity dependence on sample temperature and gas pressure can then point to the controlling mechanism. Some characteristics of Arrhenius-type curves and possible deviations from linear relation are discussed. Examples for possible surface-controlled (Ce), diffusion-controlled (Th, Ti, Zr, Hf) and interface-controlled (U) reactions are presented, as well as limited bulk intermetallic hydriding reactions. Certain symptomatic aspects of the kinetic behaviour related to some of the above mechanisms are discussed.